U.S. patent application number 11/220361 was filed with the patent office on 2007-03-08 for ceramic membranes with integral seals and support, and electrochemical cells and electrochemical cell stacks including the same.
Invention is credited to Michael J. Day, J. Michael Funk, Todd G. Lesousky, Matthew M. Seabaugh.
Application Number | 20070054169 11/220361 |
Document ID | / |
Family ID | 37830374 |
Filed Date | 2007-03-08 |
United States Patent
Application |
20070054169 |
Kind Code |
A1 |
Day; Michael J. ; et
al. |
March 8, 2007 |
Ceramic membranes with integral seals and support, and
electrochemical cells and electrochemical cell stacks including the
same
Abstract
Ceramic membranes with integral seals and support, and related
electrochemical cells and cell stacks. The membrane comprises a
thin electrolyte layer supported on a porous electrode layer which
in turn is supported on a thick ceramic support layer, preferably a
ceramic electrolyte support. The support layer is divided into a
plurality of self-supporting thin membrane regions by a network of
thicker integrated support ribs. The thin electrolyte layer and
thick ceramic support layer preferably define a sealing perimeter
surrounding the porous electrode layer.
Inventors: |
Day; Michael J.; (Dublin,
OH) ; Funk; J. Michael; (Glouster, OH) ;
Lesousky; Todd G.; (Columbus, OH) ; Seabaugh; Matthew
M.; (Columbus, OH) |
Correspondence
Address: |
Porter, Wright, Morris & Arthur LLP;IP DOCKETING, Attn: Charma Murphy
28th Floor
41 South High St.
Columbus
OH
43215-6194
US
|
Family ID: |
37830374 |
Appl. No.: |
11/220361 |
Filed: |
September 6, 2005 |
Current U.S.
Class: |
429/482 ;
429/468; 429/469; 429/495; 429/533 |
Current CPC
Class: |
H01M 4/9033 20130101;
H01M 8/0282 20130101; H01M 2300/0074 20130101; H01M 8/2435
20130101; Y02E 60/50 20130101; H01M 8/1226 20130101; H01M 2008/1293
20130101 |
Class at
Publication: |
429/030 ;
429/034 |
International
Class: |
H01M 8/12 20070101
H01M008/12; H01M 8/02 20070101 H01M008/02; H01M 8/24 20070101
H01M008/24 |
Claims
1. A ceramic membrane, comprising: a thin ceramic electrolyte
layer; an intermediate porous electrode layer supporting the thin
ceramic electrolyte layer; and a thick ceramic layer supporting the
intermediate layer, the ceramic support layer defining a plurality
of voids separated by a network of support ribs.
2. The ceramic membrane of claim 1, wherein the thin electrolyte
layer and the ceramic support layer each extend radially outwardly
beyond the perimeter of the porous electrode layer to define a
sealing perimeter that encapsulates the porous electrode layer.
3. The ceramic membrane of claim 1, wherein the porous electrode
layer defines a perimeter spaced inwardly from the perimeters of
the thin electrolyte layer and the ceramic support layer such that
the thin electrolyte layer and the ceramic support layer define a
sealing perimeter that encapsulates the porous electrode layer.
4. The ceramic membrane of claim 1, wherein the porous electrode
layer is formed by sintering a fugitive-containing ceramic
electrode material in the green state.
5. A ceramic membrane, comprising: a thin layer comprising a
ceramic electrolyte material in the green state; an intermediate
layer supporting the thin electrolyte layer, the intermediate layer
being selected from a fugitive-containing electrode material in the
green state and a porous electrode material in the green state; and
a thicker layer supporting the intermediate layer, the support
layer comprising a ceramic material in the green state and defining
a plurality of voids separated by a network of support ribs; the
tri-layer assembly being laminated and then sintered to form a
composite structure having a porous intermediate layer.
6. The ceramic membrane of claim 5, wherein the thin electrolyte
layer and the support layer each extend radially outwardly beyond
the perimeter of the porous electrode layer to define a sealing
perimeter that encapsulates the porous electrode layer.
7. The ceramic membrane of claim 5, wherein the porous electrode
layer defines a perimeter spaced inwardly from the perimeters of
the thin electrolyte layer and the ceramic support layer such that
the electrolyte layer and the ceramic support layer define a
sealing perimeter that encapsulates the porous electrode layer.
8. An electrochemical cell, comprising: the ceramic membrane of
claim 1; and a second electrode layer deposited on the thin
electrolyte surface, the second electrode layer having a polarity
opposite that of the porous electrode.
9. An electrochemical cell, comprising: the ceramic membrane of
claim 2; and a second electrode layer deposited on the thin
electrolyte surface, the second electrode layer having a polarity
opposite that of the encapsulated electrode.
10. An electrochemical cell, comprising: the ceramic membrane of
claim 3; and a second electrode layer deposited on the thin
electrolyte surface, the second electrode layer having a polarity
opposite that of the encapsulated electrode.
11. An electrochemical cell, comprising: the ceramic membrane of
claim 4; and a second electrode layer deposited on the thin
electrolyte surface, the second electrode layer having a polarity
opposite that of the porous electrode.
12. An electrochemical cell, comprising: the ceramic membrane of
claim 5; and a second electrode layer deposited on the thin
electrolyte surface, the second electrode layer having a polarity
opposite that of the porous electrode.
13. The electrochemical cell of claim, comprising: the ceramic
membrane of claim 6; and a second electrode layer deposited on the
thin electrolyte surface, the second electrode layer having a
polarity opposite that of the encapsulated electrode.
14. The electrochemical cell of claim, comprising: the ceramic
membrane of claim 7; and a second electrode layer deposited on the
thin electrolyte surface, the second electrode layer having a
polarity opposite that of the encapsulated electrode.
15. An electrochemical cell stack, comprising: a first dense
electronically conductive plate; an electrochemical cell of claim
8, the first plate being adjacent to the support surface of the
cell and in electrical contact with the porous electrode of the
cell; a second dense electronically conductive plate in electrical
contact with the electrode deposited on the thin electrolyte of the
cell; a second electrochemical cell of claim 8, the second plate
being adjacent to the support surface of the second cell and in
electrical contact with the porous electrode of the second cell;
and a third dense electronically conductive plate in electrical
contact with the electrode deposited on the thin electrolyte of the
second cell.
16. The electrochemical cell stack of claim 15, wherein at least
one dense electronically conductive plate is selected from a nickel
chrome superalloy, a ferritic stainless steel, and a lanthanum
chromite.
17. An electrochemical cell stack, comprising: a first dense
electronically conductive plate; an electrochemical cell of claim
9, the first plate being adjacent to the support surface of the
cell and in electrical contact with the encapsulated electrode of
the cell; a second dense electronically conductive plate in
electrical contact with the electrode deposited on the thin
electrolyte of the cell; a second electrochemical cell of claim 9,
the second plate being adjacent to the support surface of the
second cell and in electrical contact with the encapsulated
electrode of the second cell; and a third dense electronically
conductive plate in electrical contact with the electrode deposited
on the thin electrolyte of the second cell.
18. An electrochemical cell stack, comprising: a first dense
electronically conductive plate; an electrochemical cell of claim
10, the first plate being adjacent to the support surface of the
cell and in electrical contact with the encapsulated electrode of
the cell; a second dense electronically conductive plate in
electrical contact with the electrode deposited on the thin
electrolyte of the cell; a second electrochemical cell of claim 10,
the second plate being adjacent to the support surface of the
second cell and in electrical contact with the encapsulated
electrode of the second cell; and a third dense electronically
conductive plate in electrical contact with the electrode deposited
on the thin electrolyte of the second cell.
19. An electrochemical cell stack, comprising: a first dense
electronically conductive plate; an electrochemical cell of claim
11, the first plate being adjacent to the support surface of the
cell and in electrical contact with the porous electrode of the
cell; a second dense electronically conductive plate in electrical
contact with the electrode deposited on the thin electrolyte of the
cell; a second electrochemical cell of claim 11, the second
conductive plate being adjacent to the support surface of the
second cell and in electrical contact with the porous electrode of
the second cell; and a third dense electronically conductive plate
in electrical contact with the electrode deposited on the thin
electrolyte of the second cell.
20. An electrochemical cell stack, comprising: n electrochemical
cells of claim 8, wherein n.gtoreq.2; and n+1 dense electronically
conductive plates; wherein each of n-1 plates is adjacent to the
support of one of the n electrochemical cells and in electrical
contact with both the porous electrode of the same cell and the
electrode deposited on the thin electrolyte of another one of the n
electrochemical cells, and each of the remaining 2 plates is in
electrical contact with an outer surface of one of the outermost of
the n electrochemical cells.
21. An electrochemical cell stack, comprising: n electrochemical
cells of claim 9, wherein n.gtoreq.2; and n+1 dense electronically
conductive plates; wherein each of n-1 plates is adjacent to the
support surface of one of the n electrochemical cells and in
electrical contact with both the encapsulated electrode of the same
cell and the electrode deposited on the thin electrolyte of another
one of the n electrochemical cells and each of the remaining 2
plates is in electrical contact with an outer surface of one of the
outermost of the n electrochemical cells.
22. An electrochemical cell stack, comprising: n electrochemical
cells of claim 10, wherein n.gtoreq.2; and n+1 dense electronically
conductive plates; wherein each of n-1 plates is adjacent to the
support surface of one of the n electrochemical cells and in
electrical contact with both the encapsulated electrode of the same
cell and the electrode deposited on the thin electrolyte of another
one of the n electrochemical cells, and each of the remaining 2
plates is in electrical contact with an outer surface of one of the
outermost of the n electrochemical cells.
23. An electrochemical cell stack, comprising: n electrochemical
cells of claim 11, wherein n.gtoreq.2; and n+1 dense electronically
conductive plates; wherein each of n-1 plates is adjacent to the
support surface of one of the n electrochemical cells and in
electrical contact with both the porous electrode of the same cell
and the electrode deposited on the thin electrolyte of another one
of the n electrochemical cells, and each of the remaining 2 plates
is in electrical contact with an outer surface of one of the
outermost of the n electrochemical cells.
24. An electrochemical cell stack, comprising: a first dense
electronically conductive plate; an electrochemical cell of claim
12, the first plate being adjacent to the support surface of the
cell and in electrical contact with the intermediate layer of the
cell; a second dense electronically conductive plate in electrical
contact with the electrode deposited on the thin electrolyte of the
cell; a second electrochemical cell of claim 12, the second plate
being adjacent to the support surface of the second cell and in
electrical contact with the intermediate layer of the second cell;
and a third dense electronically conductive plate in electrical
contact with the electrode deposited on the thin electrolyte of the
second cell.
25. An electrochemical cell stack, comprising: a first dense
electronically conductive plate; an electrochemical cell of claim
13, the first plate being adjacent to the support surface of the
cell and in electrical contact with the encapsulated electrode of
the cell; a second dense electronically conductive plate in
electrical contact with the electrode deposited on the thin
electrolyte of the cell; a second electrochemical cell of claim 13,
the second plate being adjacent to the support surface of the
second cell and in electrical contact with the encapsulated
electrode of the second cell: and a third dense electronically
conductive plate in electrical contact with the electrode deposited
on the thin electrolyte of the second cell.
26. An electrochemical cell stack, comprising: a first dense
electronically conductive plate; an electrochemical cell of claim
14, the first plate being adjacent to the support surface of the
cell and in electrical contact with the encapsulated electrode of
the cell; a second dense electronically conductive plate in
electrical contact with the electrode deposited on the thin
electrolyte of the cell; a second electrochemical cell of claim 14,
the second plate being adjacent to the support surface of the
second cell and in electrical contact with the encapsulated
electrode of the second cell; and a third dense electronically
conductive plate in electrical contact with the electrode deposited
on the thin electrolyte of the second cell
27. An electrochemical cell stack, comprising: n electrochemical
cells of claim 12, wherein n.gtoreq.2; and n+1 dense electronically
conductive plates; wherein each of n-1 plates is adjacent to the
support of one of the n electrochemical cells and in electrical
contact with both the intermediate layer of the same cell and the
electrode deposited on the thin electrolyte of another one of the n
electrochemical cells and each of the remaining 2 plates is in
electrical contact with an outer surface of one of the outermost of
the n electrochemical cells.
28. An electrochemical cell stack, comprising: n electrochemical
cells of claim 13, wherein n.gtoreq.2; and n+1 dense electronically
conductive plates; wherein each of n-1 plates is adjacent to the
support of one of the n electrochemical cells and in electrical
contact with both the encapsulated electrode of the same cell and
the electrode deposited on the thin electrolyte of another one of
the n electrochemical cells and each of the remaining 2 plates is
in electrical contact with an outer surface of one of the outermost
of the n electrochemical cells.
29. An electrochemical cell stack, comprising: n electrochemical
cells of claim 14, wherein n.gtoreq.2; and n+1 dense electronically
conductive plates; wherein each of n-1 plates is adjacent to the
support of one of the n electrochemical cells and in electrical
contact with both the encapsulated electrode of the same cell and
the electrode deposited on the thin electrolyte of another one of
the n electrochemical cells and each of the remaining 2 plates is
in electrical contact with an outer surface of one of the outermost
of the n electrochemical cells.
30. A planar ceramic membrane structure for an electrochemical
cell, comprising: a dense electrolyte; a dense mechanical support
perforated by an plurality of voids; a porous electrode that
defines the active area of the cell; and a dense circumferential
region that provides an edge seal to the active area; the porous
electrode being encapsulated within the dense electrolyte, dense
mechanical support, and dense circumferential region.
31. A planar ceramic membrane structure for an electrochemical
cell, comprising: a dense electrolyte; a dense mechanical support
perforated by a plurality of voids; and a porous electrode material
that defines the active area of the cell; the dense electrolyte and
dense mechanical support cooperating to encapsulate the porous
electrode within the structure and form a dense-circumferential
region that provides an edge seal to the active area.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] Not applicable
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
[0002] Not applicable
REFERENCE TO MICROFICHE APPENDIX
[0003] Not applicable
FIELD OF THE INVENTION
[0004] The invention relates to thin film membranes of ceramic
materials with integral seals and support, electrochemical cells
including these membranes, and stacks formed from these
electrochemical cells. The disclosed membrane and resultant cell
and stack architecture are particularly well suited to applications
in which mechanical ruggedness and volumetric and gravimetric
reaction density are desirable. This invention may be useful in
electrochemical separations and catalytic reactors including but
not limited to solid oxide fuel cells and oxygen separation
membranes.
BACKGROUND OF THE INVENTION
[0005] Tubular solid oxide fuel cells (SOFCs) are the most
extensively demonstrated of the many designs proposed for SOFCs. In
these structures, a multi-layer tube is fabricated with cathode,
electrolyte, and anode layers. Tubes that are supported by anodes,
cathodes, and electrolytes each have been proposed in the
literature and demonstrated. Electrolyte- and cathode-supported
tubes, in both circular and flat tube configurations, have been
demonstrated by Westinghouse and Siemens-Westinghouse Power
Corporation. Anode-supported tubes have been demonstrated by a
range of manufacturers.
[0006] In tubular SOFCs, fuel or air is flowed down the center of
the tube, depending on whether the tube is anode- or
cathode-supported, while the complementary gas mix is flowed
outside the tube. Such tubes can have open or closed ends and are
typically sealed outside the reaction zone of the SOFC.
Conventional tubular cells typically suffer from low volumetric or
gravimetric power density because large tubes do not pack well and
have a low surface area to volume ratio.
[0007] Microtubular SOFCs, in which small-diameter (i.e., <5 mm)
tubes of electrolyte are slurry coated with cathode and anode
components, overcome some of the disadvantages of conventional
tubes. Sealing of small diameter microtubes is simpler than sealing
of conventional tubes. Microtubular cells also overcome the low
surface area to volume ratio associated with conventional tubular
cells. However, microtubular cells require complex manifolding and
electrical interconnection schemes, which makes scaling to large
power stacks difficult.
[0008] Planar SOFCs, which may be supported by either the electrode
or the electrolyte, also have been demonstrated extensively.
Electrode-supported cells have a thick electrode component that
acts as the mechanical load-bearing member of the cell and a thin
electrolyte layer. This design reduces electrolyte ohmic resistance
in the cell and allows operation at intermediate temperatures
(e.g., T<800.degree. C.). Electrode-supported SOFCs typically
are produced by co-sintering the support electrode material and a
thin coating of electrolyte material. The electrode support is
typically tape cast, calendared, or slip cast, although other
preparation methods have been demonstrated. The thin electrolyte
can be deposited in a number of ways, including but not limited to
lamination of electrolyte tape, screen printing, calendaring, and
spray deposition. Electrode-supported cells preferably have an
electrolyte that is less than twenty microns in thickness after
sintering and well-adhered to the electrode support.
[0009] Electrode-supported planar SOFCs include both cathode- and
anode-supported cells. Cathode-supported cells have the potential
to be lightweight and lower in cost than anode-supported cells.
However, processing of cathode-supported cells is difficult because
the co-firing of most cathode materials in contact with an
electrolyte produces insulating intermediate compounds.
Anode-supported electrolytes are perhaps the most widely evaluated
cell geometry for low temperature operation. Processing of
anode-supported cells is comparatively easy because sintering
temperatures in excess of 1300.degree. C. can be used to achieve
dense electrolytes without concern for interaction between the
anode material and the supported electrolyte.
[0010] Planar anode-supported cells are particularly attractive for
mass market, cost driven applications because of their high areal
power density and their advantageous packing efficiency.
Performance of anode-supported cells at 700.degree. C. has been
demonstrated to be over 1 W/cm.sup.2 in small cells at low fuel
utilization. With appropriate seal and interconnect technology,
power densities greater than 0.4 W/cm.sup.2 have been reported for
anode-supported cell stacks. However, anode-supported cells are not
without drawbacks. When conventional nickel
oxide/yttrium-stabilized zirconia (NiO/YSZ) composites are used as
support materials, the reduction of NiO to nickel metal creates
stress in the electrolyte layer, which may cause considerable
deformation during this reduction process. Operating planar
anode-supported cells at high power density and high fuel
utilization also is difficult; the thick porous layer prevents
rapid diffusion of steam away from the electrolyte and results in
increased cell area-specific resistance (ASR) at high current
density.
[0011] Electrolyte-supported planar cells have an electrolyte layer
that provides the mechanical strength of the cell. The electrolyte
layer can be produced by tape casting or other methods. Electrodes
typically are deposited on the electrolyte layer by screen printing
or spray coating and fired in a second step. To achieve strong
electrode adhesion, the ink particle size, composition, and surface
area must be tailored to the target firing temperature and
controlled during fabrication. Electrodes can be sintered in two
separate steps or simultaneously, depending upon the requisite
temperatures for the cathode and anode. In many cases, the anode
ink is fired first because it is more refractory and more difficult
to sinter, and the cathode ink is applied and fired in a second
step at a lower temperature to minimize the chemical interaction
between the electrolyte and cathode.
[0012] Electrolyte-supported cells offer numerous advantages in the
production of SOFCs. The sealing of electrolyte-supported cells is
simpler than electrode-supported planar cells because a dense
electrolyte perimeter can be preserved during cell processing,
which provides a dense, smooth surface for sealing operations.
Electrolyte-supported cells also have good stability during
reduction. Because only a thin layer of anode ink is affected by
the reduction process, this process generally has little impact on
cell mechanical stability. The gas diffusion path in and out of the
thinner anode layer is short, making fuel and steam diffusion
limitations less of a concern.
[0013] However, under identical operating conditions,
electrolyte-supported cells often exhibit much higher area-specific
resistance values than electrode-supported cells because the
electrolyte is more resistive than the anode or cathode materials.
To compensate for this higher area-specific resistance, the
operating temperature for electrolyte-supported cells generally is
higher than anode-supported cells using the same materials set. The
higher operating temperature of the electrolyte-supported cells can
be a drawback, particularly for developers wishing to use metallic
interconnect materials.
[0014] In spite of more than thirty years of continuous research in
the area of SOFCs, these systems remain far from commercialization.
Until SOFC cells are developed that address the shortcomings of
existing cell structures, it will be difficult for SOFCs to
overcome the commercialization barriers and compete with
conventional energy production routes. Considering planar cells in
particular, a cell that delivers high performance, high mechanical
strength, and easier sealing than current electrolyte- or
anode-supported cells is essential in advancing commercialization
of SOFCs.
SUMMARY OF THE INVENTION
[0015] The present invention provides a mechanically robust
supported ceramic membrane structure. This structure provides the
advantages of both electrolyte-supported cells (a dense sealing
perimeter, high mechanical strength, and thin electrode layers that
avoid diffusion limitation of performance) and electrode-supported
cells (low ohmic contribution of the electrolyte layer and the
potential for low temperature operation) without the drawbacks of
these conventional cells. The structure is useful in the
fabrication of electrochemical cells; when appropriate electrode
materials are selected, the cell may be used as a fuel cell, oxygen
separator, or other electrochemical device.
[0016] The structure comprises a very thin electrolyte (less than
50 microns) supported by a thin layer of a porous electrode
material (less than 100 microns). The two layers form a thin
membrane reactor that is supported by a mesh-like mechanical
support layer. The membrane structure of the present invention may
be prepared by laminating a thin electrolyte layer in the green
state with an electrode layer in the green state. The mechanical
support may be attached by laminating a third, thicker ceramic
layer to the bi-layer, also in the green state. This mechanical
support has been preformed to provide a meshed network of support
ribs.
[0017] Preferably, the thin electrolyte layer and ceramic support
layer extend radially outwardly beyond the perimeter of the
electrode layer to form a dense sealing perimeter. This sealing
perimeter encapsulates the electrode layer. The electrode layer is
sintered to the adjacent surfaces of the thin electrolyte and
ceramic support layers within the interior of the structure and
does not extend to the outer surface of the structure. The dense
sealing perimeter formed by the thin electrolyte layer and the
ceramic support layer is particularly well suited for stack
fabrication.
[0018] The thin electrolyte layer may be prepared by tape casting
or other processes that result in a layer having a thickness of
less than 50 microns after firing. The electrode layer may be
prepared by tape casting or other processes that result in a layer
having a thickness of less than 100 microns after firing. The
electrode layer preferably is porous. The electrode material may be
porous; alternatively, the electrode material in the green state
may contain a fugitive material, resulting in pore formation upon
sintering of the electrode layer. The thicker support layers may be
produced by punching or cutting green sheets produced by tape
casting; by conventional casting methods including but not limited
to slip casting or gel casting; by dry or semi-dry pressing using
isostatic or uniaxial presses; or by printing the pattern by solid
freeform fabrication or similar high solids extrusion processes.
Thin layers of ceramic can also be laminated in the green state to
form thicker support layers.
[0019] The preferred method for lamination, described herein, is
the use of pressure and temperature to bond the three layers by
heating the green ceramic tape above the glass transition
temperature of the polymer component to achieve intimate contact
and bonding between the layers. The electrolyte, electrode and
support layers are compressed at temperatures below 100.degree. C.
to produce a laminate structure. The laminates are subsequently
heated to 600.degree. C. to remove the polymeric binder. The
resultant structures are sintered at temperatures above
1000.degree. C. to densify the structure and provide adherence and
cohesion between layers.
[0020] The architecture of electrochemical cells including the
ceramic membranes of the present invention offer significant
advantages in processing, electrochemical performance, mechanical
integrity, the sealing of stacks, and the facility of gas flow
compared to electrode-supported cells. This architecture also
provides a means of translating the advantages of thin electrolytes
to a robust electrolyte-supported structure.
[0021] The planar structure of the present invention also provides
a flexible platform for a range of electrochemical cells by the
selection of appropriate electrode layers (either anode or cathode
layers can be considered for the porous layer) and corresponding
vehicles such as screen printing inks. The simple planar geometry
of the cell also allows the use of existing electrode materials and
processes developed for both electrode- and electrolyte-supported
cells. The membranes and cells of the present invention are
particularly well-suited to large volume manufacturing and low cost
processes.
[0022] The mesh-like mechanical support creates a macroscopic
texture on at least one side of the disclosed cells and, together
with the electrolyte layer, serves to encapsulate the edges of the
electrode layer. In the disclosed examples, the anode layer is
encapsulated on this textured side of the structure and the cathode
is deposited on the opposing side. Conceptually equivalent cells
could be produced in which the cathode layer supports the
electrolyte and is encapsulated by the electrolyte and support,
while the anode layer is deposited on the untextured side of the
cell.
[0023] The dense seal perimeter of the present invention is
particularly well suited for stack fabrication. Fuel cell stacks
can be produced by interleaving electrochemical cells formed using
disclosed structure with dense interconnect plates. The
interconnect plates separate the air and fuel streams while
providing an electrical series connection between the cells. The
strength and flexibility of the proposed membrane structure allows
the resultant cells to achieve cell-to interconnect conformance
during stack assembly by applying small compressive forces; good
contact along the perimeter improves stack sealing while good area
contact between the cells and the interconnect reduces stack
resistance.
[0024] In one embodiment, the invention provides a ceramic membrane
comprising a thin ceramic electrolyte layer, an intermediate porous
electrode layer supporting the thin ceramic electrolyte layer, and
a thick ceramic layer supporting the intermediate layer; the
ceramic support layer defines a plurality of voids separated by a
network of support ribs. The thin electrolyte layer and the ceramic
support layer each may extend radially outwardly beyond the
perimeter of the porous electrode layer to define a sealing
perimeter that encapsulates the porous electrode layer. The porous
electrode layer also may define a perimeter spaced inwardly from
the perimeters of the thin electrolyte layer and the ceramic
support layer such that the thin electrolyte layer and the ceramic
support layer define a sealing perimeter that encapsulates the
porous electrode layer. The porous electrode layer may be formed by
sintering a fugitive-containing ceramic electrode material in the
green state.
[0025] In another embodiment, the invention provides a ceramic
membrane comprising a thin layer comprising a ceramic electrolyte
material in the green state, an intermediate layer supporting the
thin electrolyte layer, and a thicker layer supporting the
intermediate layer. The support material comprises a ceramic
material in the green state and defining a plurality of voids
separated by a network of support ribs; the intermediate layer
comprises a fugitive-containing electrode material in the green
state or a porous electrode material in the green state. The
tri-layer assembly is laminated and then sintered to form a
composite structure having a porous intermediate layer. The thin
electrolyte layer and the support layer each may extend radially
outwardly beyond the perimeter of the porous electrode layer to
define a sealing perimeter that encapsulates the porous electrode
layer. The porous electrode layer also may define a perimeter
spaced inwardly from the perimeters of the thin electrolyte layer
and the ceramic support layer such that the electrolyte layer and
the ceramic support layer define a sealing perimeter that
encapsulates the porous electrode layer.
[0026] Other embodiments of the invention include electrochemical
cells comprising any of the above-described ceramic membranes and a
second electrode layer deposited on the thin electrolyte surface.
The second electrode layer has a polarity opposite that of the
porous or encapsulated electrode.
[0027] The invention also provides electrochemical cell stacks that
include the above-described electrochemical cells. In one
embodiment, the electrochemical cell stack comprises a first dense
electronically conductive plate, an electrochemical cell with the
first plate adjacent to its support surface and in electrical
contact with its porous or encapsulated electrode, a second dense
electronically conductive late in electrical contact with the
electrode deposited on its thin electrolyte, a second
electrochemical cell with the second plate adjacent to its support
surface and in electrical contact with its porous or encapsulated
electrode, and a third dense electronically conductive plate in
electrical contact with the electrode deposited on the second
cell's thin electrolyte. At least one dense electronically
conductive plate may be nickel chrome superalloy, a ferritic
stainless steel, or a lanthanum chromite.
[0028] In another embodiment, the electrochemical cell stack
comprises n electrochemical cells, as described above, wherein
n.gtoreq.2, and n+1 dense electronically conductive plates. Each of
n-1 plates is adjacent to the support of one of the n
electrochemical cells and in electrical contact with both the
porous or encapsulated electrode of the same cell and the electrode
deposited on the thin electrolyte of another one of the n
electrochemical cells. Each of the remaining 2 plates is in
electrical contact with an outer surface of one of the outermost of
the n electrochemical cells.
[0029] The invention also provides planar ceramic membrane
structures for an electrochemical cell. In one embodiment, the
planar ceramic membrane structure comprises a dense electrolyte, a
dense mechanical support perforated by an plurality of voids, a
porous electrode that defines the active area of the cell, and a
dense circumferential region that provides an edge seal to the
active area. The porous electrode is encapsulated within the dense
electrolyte, dense mechanical support, and dense circumferential
region. In another embodiment, the planar ceramic membrane
structure comprises a dense electrolyte, a dense mechanical support
perforated by a plurality of voids, and a porous electrode material
that defines the active area of the cell. The dense electrolyte and
dense mechanical support cooperate to encapsulate the porous
electrode within the structure and form a dense circumferential
region that provides an edge seal to the active area.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] These and further objects of the invention will become
apparent from the following detailed description.
[0031] FIG. 1 is an exploded schematic view of an electrochemical
cell of the present invention, particularly illustrating the
membrane structure.
[0032] FIG. 2 is a cutting pattern for the support layer of the
membrane structure of Example 1.
[0033] FIG. 3. is an exploded schematic view of the components
during various steps in the assembly and processing of one
embodiment of the invention, particularly illustrating the cross
sectional architecture of the structure and the sequence of layer
fabrication.
[0034] FIG. 4 is the final cutting pattern (dashed line) of the
membrane structure of Examples 1.
[0035] FIG. 5 is a photograph of the composite structure produced
after sintering the membrane structure of Example 1.
[0036] FIG. 6 is a cutting pattern for the support layer of the
membrane structure of Example 2.
[0037] FIG. 7 is the final cutting pattern (dashed line) of the
membrane structure of Example 2.
[0038] FIG. 8 is a cutting pattern for the support layer of the
membrane structure of Example 3.
[0039] FIG. 9 is the final cutting pattern (dashed line) of the
membrane structure of Example 3.
[0040] FIG. 10 is a cutting pattern for the support layer of the
membrane structure of Example 4.
[0041] FIG. 11 is the final cutting pattern (dashed line) of the
membrane structure of Example 4.
[0042] FIG. 12 is a photograph of the composite structure produced
after sintering the membrane structure of Example 4.
[0043] FIG. 13 is the cutting pattern for the support layer of the
membrane structure of Example 5.
[0044] FIG. 14 is the final cutting pattern (dashed line) of the
membrane structure of Example 5.
[0045] FIG. 15 is a graph of the fuel cell performance of the cell
of Example 6 at 850 and 750.degree. C.
[0046] FIG. 16 is a graph of comparative fuel cell performance at
750.degree. C. for the cell of Example 6 and conventional fuel
cells.
DETAILED DESCRIPTION OF CERTAIN PREFERRED EMBODIMENTS
[0047] The present invention provides a membrane structure useful
in electrochemical cells. When appropriate electrode materials are
included in the structure and applied to the exposed electrolyte
surface, the structure may form a fuel cell, oxygen separator, or
other electrochemical device. The resultant electrochemical cells
may be assembled and electrically connected to form stacks.
[0048] FIG. 1 shows an exploded schematic view of an
electrochemical cell including the membrane structure of the
present invention. The membrane structure comprises a mechanical
support layer A, a porous electrode layer B, and an electrolyte
layer C. An electrode D having a polarity opposite that of
electrode layer B may be applied to the electrolyte layer C by
screen printing or other conventional methods to form an
electrochemical cell. The membrane structure of the present
invention yields cells having the advantages of both
electrolyte-supported cells (a dense sealing perimeter, high
mechanical strength, and thin electrode layers that avoid diffusion
limitations) and electrode-supported cells (low ohmic contribution
of the electrolyte layer and potential for low temperature
operation) without the drawbacks of these conventional designs.
[0049] The electrolyte membrane (layer C of FIG. 1) is the thinnest
of the three layers, preferably less than 50 microns thick. The
porous electrode (layer B of FIG. 1) preferably is less than 100
microns thick. Preferably, the porous electrode layer in the
composite structure is less than 70% dense. The green electrode
layer may contain pores or the green electrode layer may contain a
fugitive material such that pores are formed in the electrode layer
after sintering.
[0050] The mechanical support layer preferably is a relatively
thick (50-250 micron thick) mesh-like component, preferably a
ceramic material including but not limited to a ceramic electrolyte
material. Preferably, the material of the support layer is cast,
cut, or punched to form a mesh pattern. The mesh pattern may be any
network providing a desired ratio of exposed to supported area, of
any shape or pattern. Examples of mesh patterns are shown in FIGS.
2, 6, 8, and 10, but other patterns also may yield satisfactory
results.
[0051] A mesh-like pattern of ribs separated by voids is formed in
the support layer. The ratio of void area to rib area preferably is
.gtoreq.60/40. The ribs preferably have a substantially uniform
thickness, generally in the range of 30-300 microns. The voids in
the structure preferably have a substantially uniform diameter,
generally between 0.05 mm and 10 mm, and extend through the
thickness of the support layer. The voids may define substantially
identical shapes or may vary in size and/or shape across a cell to
improve overall void packing density. Preferably, the voids define
regular polygons, although voids defining circles and other shapes
also may be used. Hexagonal voids are particularly preferred
because they reduce the number of stress concentrating angles,
achieve a highly equilibrated stress distribution in the plane
during sintering and provide a strong support with a high ratio of
exposed to supported area. In a preferred embodiment, a cross
section of the mesh support layer in a plane parallel to the thin
electrolyte layer defines a honeycomb-like structure, shown, e.g.,
in FIG. 2, which provides excellent access to the dense thin
electrolyte layer.
[0052] Each layer may comprise one or more sheets of a ceramic,
cermet, or metal material in the green state. Any number of sheets
may be used to achieve the desired thickness of each electrolyte
layer. The use of multiple sheets in the electrolyte layer
minimizes the risk of critical continuous defects (e.g., pinholes)
through an electrolyte layer, though it makes achieving thin
electrolyte layers difficult. The use of multiple sheets in the
electrode layer also offers the advantage that sheets of dissimilar
compositions can be used to build a composite support layer or
functionally grade the electrode to achieve compatibility with a
dissimilar electrolyte layer, provide enhanced catalytic or
electrochemical function, or other desirable function. The
mechanical support layer can be produced in a single layer or by
consolidating a number of layers to achieve the desired thickness.
Lamination increases the complexity of fabrication, but offers a
path to laminar composite approaches, which can provide strength
enhancements.
[0053] The composition of the three layers may, and likely will,
differ. A more important aspect of the composition of the layers is
that the shrinkage of the layers upon thermal processing is similar
and no deleterious chemical reactions occur between the respective
layers. In some cases, for example, it may be preferred to have a
mechanically strong, relatively poor conductor in the mechanical
support, a highly conductive electron conductor in the porous
layer, and a mechanically weaker but excellent ionic conductor in
the electrolyte.
[0054] Preferably, the thin electrolyte layer and the thicker
support layer each is selected from a partially stabilized zirconia
composition. The thin electrolyte layer preferably is a
scandia-stabilized zirconia composition. Other compositions,
including but not limited to doped cerium oxides, doped zirconium
oxides, lanthanum gallates, bismuth oxide ceramics, other ionic or
mixed conducting ceramics, or mixtures of the above, also may yield
satisfactory results. The mechanical support layer preferably is a
partially-stabilized zirconia composition, more preferably, a 6 mol
% scandia-stabilized zirconia composition or 3 mol %
yttria-stabilized composition. Other compositions, including but
not limited to doped cerium oxides, dopes zirconium oxides,
lanthanum gallates, bismuth oxide ceramics, other ionic or mixed
conducting ceramics, metals, cermets or mixtures of the above, also
may yield satisfactory results.
[0055] The electrode layer may comprise a wide range of materials
and composites. In one embodiment, the electrode layer may be
formed from a composition suitable for performing the anode
function or a fuel cell or other electrochemical cell and
conducting current under reducing conditions. Particularly
preferred electrodes in this embodiment include NiO-zirconia
composites, NiO-ceria composites, other cermets, metals or ceramics
with the above-described properties, and combinations thereof. In
another embodiment, the electrode layer may be formed from a
composition suitable for performing the cathode function for a fuel
cell or other electrochemical cell. Particularly preferred
electrodes in this embodiment may comprise electronic or mixed
electronic-ionic conductors including lanthanide manganites,
ferrites, cobaltites or other conducting ceramics, or cermets or
metallic materials that can be cosintered in an appropriate manner
with the electrolyte or support.
[0056] Preferably, the electrode layer in each of the
above-described embodiments is porous, with the porosity providing
a path for gas diffusion to the electrolyte/electrode interface, a
critical region of electrochemical reaction in the resultant
electrochemical cell. As described above, this may be accomplished
using either a porous electrode layer or a fugitive-containing
electrode material in the green state.
[0057] The thin electrolyte layer may be prepared by tape casting
or other processes that yield a sheet or stack of sheets having a
thickness of less than 50 microns after firing. The thin
electrolyte layer may comprise a stack of at least two-sheets in
the green state. The electrode layer may be prepared by tape
casting or other processes that yield a sheet or stack of sheets
having a thickness of less than 100 microns after firing, with
microscopic porosity in the layer of more than 30% of the fired
layer. Like the thin electrolyte layer, the thin electrode layer
may comprise a stack of at least two-sheets in the green state.
[0058] The mechanical support layer may be produced by punching or
cutting green sheets produced by tape casting; by conventional
casting methods including but not limited to slip casting or gel
casting; by dry or semi-dry pressing using isostatic or uniaxial
presses; or by printing the pattern by solid freeform fabrication
or similar high solids extrusion processes. It also may be possible
to produce voids in the thicker support layer by the burn-off of
fugitive materials contained within the support layer. For tape
cast sheets, the support layer preferably comprises a single sheet,
although two or more sheets (e.g., three and four sheets) also may
be used. Multiple sheets may be laminated, e.g., at 80.degree. C.
and 12 MPa. The laminated may then be cut using a laser cutting
system or similar device to form a network of interconnected ribs
separated by voids. This may be accomplished, for example, by
cutting the green perform sheet by laser cutting. A pattern, such
as the hexagonal pattern shown in FIG. 2, may be cut into the
laminate including an uncut perimeter area to allow effective
sealing with the thin electrolyte layer and encapsulate the
electrode layer. The cut-out laminate may then be set aside.
[0059] In one embodiment, the membrane of the present invention may
be prepared by laminating the thin electrolyte layer in the green
state to the electrode layer and subsequently to the mechanical
support layer in the green state. The process is shown
schematically in FIG. 3. In Step 1 of FIG. 3, a cross section view
of the electrolyte (top), electrode (middle), and mechanical
support (bottom) are shown.
[0060] The pieces are laminated by applying isostatic pressure to
the stack of tapes, causing the electrolyte layer to deform and
encapsulate the electrode layer, as shown in Step 2 of FIG. 3.
Although the drawings of FIG. 3 are not to scale, it is important
to note that the electrode layer is smaller radially than the
electrolyte and mechanical support layers; the thin electrolyte
layer and thick ceramic support layers extend radially outwardly
beyond the perimeter of the electrode layer such that after
lamination, the electrode layer is encapsulated within the thin
electrolyte and ceramic support layers and does not extend to the
outer surface of the structure. As used herein, "perimeter" refers
to the outer edge of a layer, regardless of the shape of the layer.
By applying isostatic force to the laminate at temperatures above
the glass transition of the tape binder, the electrolyte deforms
uniformly, avoiding cracking or unequal stress distributions.
Preferably, the portions of the laminated thin electrolyte and
ceramic support layers extending beyond the perimeter of the
electrode layer form a dense sealing perimeter useful in the
assembly of electrochemical cell stacks.
[0061] The laminate can then be heat treated to remove the tape
casting binders from the laminae, and then sintered to produce a
ceramic part with a dense electrolyte layer. During this heat
treatment, all the lamina shrink to produce a fired part that is
smaller than the green part while maintaining the architecture of
the component, as shown in Step 3 of FIG. 3. The sintering of the
cell creates a dense ceramic continuum at the perimeter that
encapsulates the porous electrode, completely eliminating any
lateral gas diffusion paths out of the electrode region. Step 4 of
FIG. 3 shows the deposition of the opposite electrode, which
completes the electrochemical cell.
[0062] The preferred lamination method uses pressure and
temperature to bond the two layers by heating above the glass
transition temperature of the polymer component of the green
support layer to achieve intimate contact and bonding between the
layers. The layers typically are compressed at temperatures below
100.degree. C. to produce a laminate structure. After lamination,
the laminate may be trimmed using a laser cutter. A pattern, such
as that shown in FIG. 2, may be cut out of the laminate, e.g.,
using a 30-watt laser, leaving a sealing border of between 0.1 and
3 cm around the pattern of shapes. The laminate is then heated to
.about.600.degree. C. to remove the polymeric binder. The resultant
structure is sintered at temperatures above 1000.degree. C. to
densify the structure and provide adherence and cohesion
layers.
[0063] Electrochemical cells may be prepared from the laminate
membrane structure of the present invention by applying appropriate
electrode materials to the exposed electrolyte surface. This may be
accomplished, for example, by screen printing of electrode inks or
other conventional electrode application methods. In the case of a
cell with an anode layer encapsulated between the electrolyte and
the support, the cathode materials should be a composition suitable
for performing the cathode function for a fuel cell or other
electrochemical cell, such as electronic or mixed electronic-ionic
conductors such as the lanthanide manganites, ferrites, cobaltites
or other conducting ceramics, or cermets or metallic materials, or
mixtures thereof. For a cell with an encapsulated cathode layer,
anode materials for the exposed electrolyte surface may be selected
from NiO-zirconia composites, NiO-ceria composites, or other
cermets, metals or ceramics, or composites thereof suitable for
performing the anode function for a fuel cell or other
electrochemical cell and conducting current under reducing
conditions.
[0064] Screen printing is a preferred method of electrode
deposition on the exposed electrolyte, although other conventional
methods may be used. A typical cathode electrode is prepared by
depositing a first layer of an electrochemically active material
(such as a lanthanum manganite and gadolinium-doped ceria powder
mixture dispersed in an organic vehicle) and then depositing a
second ink layer (such a pure lanthanum manganite powder dispersed
in an organic vehicle) as a "current collector." After sequentially
printing and drying the two layers, the cathode is sintered at a
temperature of 1150.degree. C. For anode electrodes, an
electrochemically active anode interlayer ink (for example a finely
divided NiO and a gadolinium-doped ceria powder mixture dispersed
in an organic vehicle), will first be deposited and then a second
electrically conductive ink layer (for example a more coarsely
divided NiO and yttrium-stabilized zirconia powder mixture
dispersed in an organic vehicle) would be deposited on top of the
interlayer. The second layer serves as a high conductivity "current
collector" layer. The layers preferably are deposited by applying
the ink formulations by screen printing or other conventional
application method, including but not limited to aerosol spray
deposition, painting, and stencil or transfer printing processes.
After sequentially depositing and drying the two layers, the
electrode is sintered to a temperature of 1300.degree. C.
[0065] Electrochemical cell stacks may be prepared from the
resulting electrochemical cells by interleaving the cells with
conventional dense interconnect plates of an electrically
conducting material. The dense plates serve to separate air and
fuel streams while providing an electrical series connection
between the cells. The plates may be formed from a dense material
that is conductive in both oxidizing and reducing atmospheres,
including but not limited to an electronically conductive dense
ceramic material, lanthanum chromite, a nickel chromic superalloy,
and a ferritic stainless steel.
[0066] An electrochemical stack maybe formed from a minimum of two
self-supporting membranes or electrochemical cells and three
plates, with the first plate having an inner face adjacent to the
support side of the first membrane or cell, the second plate having
one face adjacent to the thin electrolyte side of the first
membrane or cell and the opposing face adjacent to the support side
of the second membrane or cell, and the third plate having an inner
face adjacent to the thin electrolyte side of the second membrane
or cell. Additional units may be added to the stack with the number
of membranes or cells being equal to n and the number of plates
being equal to n+1.
[0067] A stack may be prepared by connecting the membranes or cells
to the plates with a contact paste. The contact paste may penetrate
the voids in the support to provide electrical contact between the
plates and the encapsulated electrode. The contact paste may
comprise a conducting ceramic material such as a lanthanum
chromite, a cermet such as NiO/YSZ, or a metal, such as platinum or
silver.
[0068] The dense seal perimeter allows effective stack sealing with
the application of small compressive forces. The flexibility and
strength of the membranes structure allows effective flattening of
the structure at lower pressure. The conformance required for
effective mechanical sealing may be achieved with the external
application of a lower mechanical load to the stack.
[0069] Examples of membrane structures and electrochemical cells
according to the present invention are described below. These
examples are intended to illustrate and assist in understating the
invention but not to limit the scope of the invention to the
described examples.
EXAMPLE 1
Preparation of Membrane Structure I
[0070] The tri-layers were constructed with electrolyte and support
tapes prepared with 6 mol % scandium-stabilized zirconia powder
(initial SSA=8.704 m.sup.2/g). The 6ScSZ tapes for the support
structure were prepared by a conventional tape casting method and
had a thickness of approximately 45 microns in the green state. The
tape was cut into 15.times.15 cm sheets. The sheets were stacked on
top each other, five sheets per stack. The resulting five-sheet
stack was laminated at 80.degree. C. and 12 MPa. The pattern shown
in FIG. 2 was then cut in the laminate using a laser cutting
system. The cut-out laminate was set aside.
[0071] The porous anode layer was constructed with cast tapes
prepared with a nickel oxide and yttria-stabilized zirconia powder
mixture. This mixture was made using 60 mol % NiO powder (Novamet)
and 40 mol % yttria-stabilized zirconia (Unitec). The thickness of
the dry tape was 45 microns. The sheets were then cut by hand to 15
cm.times.15 cm, moved to a 30-watt laser cutting system, and cut to
form a circle with a diameter 0.5 cm greater than the mesh pattern
shown in FIG. 4.
[0072] The 6ScSZ electrolyte tapes for the thin electrolyte layer
were prepared by a conventional tape casting method. The thickness
of the electrolyte tape was 45 microns. The tape was cut into
15.times.15 cm sheets.
[0073] For lamination of the structure, the electrolyte tape was
placed on an aluminum setter covered with Mylar. The anode layer
was stacked on the electrolyte layer, centered on the electrolyte
sheet. The cut-out support laminate was then placed on top of the
anode sheet, again centered to provide a uniform overlap of
electrolyte and mechanical support, such that when laminated, the
electrode layer would be encapsulated by the overlapping
electrolyte and support layers. The stack was laminated at
80.degree. C. and 12 MPa. After lamination, the final part was cut
out of the laminate based on the pattern shown by the dashed line
in FIG. 4. The resultant component was sintered at 1400.degree. C.
for two hours to densify the electrolyte and support layers. A
photograph of the resultant component is shown in FIG. 5.
EXAMPLE 2
Preparation of Membrane Structure II
[0074] The tri-layers were prepared, cast and laminated as
described in Example 1. The support laminate was laser cut to
produce the pattern shown in FIG. 6. The cut-out laminate was set
aside. The two-sheet electrolyte-electrode stack was prepared as
described in Example 1 and laminated with the cut-out support, also
as described in Example 1. The final part was cut out of the
laminate using the pattern shown by the dashed line in FIG. 7. The
resultant component was sintered at 1400.degree. C. for two hours
to densify the electrolyte and support layers.
EXAMPLE 3
Preparation of Membrane Structure III
[0075] The tri-layers were prepared, cast and laminated as
described in Example 1. The support laminate was laser cut to
produce the pattern shown in FIG. 8. The cut-out laminate was set
aside. The two-sheet electrolyte-electrode stack was prepared as
described in Example 1 and laminated with the cut-out support, also
as described in Example 1. The final part was cut out of the
laminate using the pattern shown by the dashed line in FIG. 9. The
resultant component was sintered at 1400.degree. C. for two hours
to densify the electrolyte and support layers.
EXAMPLE 4
Preparation of Membrane Structure IV
[0076] The tri-layers were prepared, cast and laminated as
described in Example 1. The support laminate was laser cut to
produce the pattern shown in FIG. 10. The cut-out laminate was set
aside. The two-sheet electrolyte-electrode stack was prepared as
described in Example 1 and laminated with the cut-out support, also
as described in Example 1. The final part was cut out of the
laminate using the pattern shown by the solid line in FIG. 11.
After sintering at 1400.degree. C. for 2 hours, the component shown
in FIG. 12 was produced.
EXAMPLE 5
Preparation of Membrane Structure IV
[0077] The tri-layers were prepared, cast and laminated as
described in Example 1. The support laminate was laser cut to
produce the pattern shown in FIG. 13. The cut-out laminate was set
aside. The two-sheet electrolyte-electrode stack was prepared as
described in Example 1 and laminated with the cut-out support, also
as described in Example 1. The final part was cut out of the
laminate using the pattern shown by the dashed line in FIG. 14. The
resultant component was sintered at 1400.degree. C. for two hours
to densify the electrolyte and support layers.
EXAMPLE 6
Electroding and Testing of Mesh-Supported Cell
[0078] A solid oxide fuel cell was prepared using a membrane
structure produced as described in Example 1. A Sr-doped lanthanum
manganite/Gd-doped ceria composite cathode was applied by paint
roller on the exposed electrolyte membrane directly opposite the
sintered anode. The cathode was sintered at 1100.degree. C. to
achieve good adherence. Platinum meshes were attached to the anode
side of the cell using an NiO ink to serve as the anode current
collector. Silver mesh was attached to the cathode side of the cell
using a Sr-doped lanthanum manganite ink to serve as the cathode
current collector. Alumina felt seals were cut to form a perimeter
1.5 cm wide that enclosed the anode and cathode active areas. The
alumina felts were saturated with an aqueous slurry of alumina
powder to improve the density of the seal material and prevent gas
leakage.
[0079] The cells was heated to 850.degree. C. under air on the
cathode side and nitrogen gas on the anode side. The cell exhibited
a high open circuit voltage in N.sub.2 and was subsequently reduced
by substituting hydrogen for nitrogen in the anode gas stream over
a one-hour period. At the end of the reduction process, the cell
was initially fed 350 sccm H.sub.2 to the anode side and 1.6 slpm
air to the cathode side. A measurement of the cell voltage as a
function of current density was taken and the data plotted in FIG.
15. The cell was cooled to 750.degree. C. and the voltage measured
as a function of current density for various fuel dilutions. The
slope of the voltage vs. current density curve was calculated and
divided by the active area of the cell to determine the
area-specific resistance (ASR) of the cell. The ASR values of the
test cell was calculated to be 0.279 ohm-cm.sup.2 at 850.degree. C.
and 0.420 ohm-cm.sup.2 at 750.degree. C.
[0080] The impact of the cell structure on performance can be
appreciated more clearly in FIG. 16, which shows comparative data
of three SOFC components. The three cells had similar electrode
compositions and were tested under equivalent fuel utilization and
temperature (750.degree. C.) conditions. The primary difference
between the samples is the architecture, particularly the thickness
of the electrolyte and the intimacy of the electrode-electrolyte
interface. As shown in FIG. 15, a conventional
electrolyte-supported cell (120 microns thick green tape) has a
relatively high ASR value of 1.01 ohm-cm.sup.2. The thinner
electrolyte-supported cell (45 microns thick green tape) shows
improved performance, with an ASR value of 0.753 ohm-cm.sup.2. The
disclosed architecture with the same electrolyte thickness (45
microns thick green tape) but the most intimate contact between
anode and electrolyte, demonstrates the best performance of the
three, with an ASR value of 0.421 ohm-cm.sup.2. Given that all
three cells are flexible with good mechanical strength and dense
sealing perimeters, the advantages of the disclosed technology are
evident.
[0081] The preferred embodiment of this invention can be achieved
by many techniques and methods known to persons who are skilled in
this field. To those skilled and knowledgeable in the arts to which
the present invention pertains, many widely differing embodiments
will be suggested by the foregoing without departing from the
intent and scope of the present invention. The descriptions and
disclosures herein are intended solely for purposes of illustration
and should not be construed as limiting the scope of the present
invention which is described by the following claims.
* * * * *